Partially blocked pedical screw ii

ABSTRACT

A polyaxial pedicle screw has a screw shaft and a screw head with a substantially spherical base shape. The polyaxial pedicle screw is mounted in a receiving sleeve or area. An insertion sleeve can be pressed against the screw head to determine a relative pivot position between the receiving sleeve or area and the screw shaft. Two pivot guide or pivot restriction units each have at least one extension projecting radially and in the longitudinal direction of the screw beyond the spherical base shape of the screw head. The extensions form stops on their radial outer surfaces and are positioned to permit relative pivoting of the screw shaft and the area or receiving sleeve only in one pivoting plane, and to abut in a supporting manner against a radially inner circumferential side of the area or receiving sleeve during relative pivoting in another pivoting plane.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the national phase entry of InternationalApplication No. PCT/EP2020/066476, filed Jun. 15, 2020, and claimspriority to German Application No. 10 2019 116 374.9, filed Jun. 17,2019. The contents of International Application No. PCT/EP2020/066476and German Application No. 10 2019 116 374.9 are incorporated byreference herein in their entireties.

FIELD

The present disclosure relates to a partially blocked pedicle screw (inshort: screw) of the polyaxial type having a screw shaft and a screwhead integrally connected thereto, which is rotatably and/or pivotablysupported or received in a receiving sleeve (tulip).

BACKGROUND

Pedicle screws, in particular of the polyaxial type, are used insurgical procedures on the spinal column in order to fix the position ofseveral vertebrae in relation to each other. The screws are each screwedinto the pedicles of a vertebra and are connected via a connecting rodinserted into the screw heads or in the tulips connected to them.Depending on the application, screws with different degrees of freedomwith regard to a relative movement between a tulip and a screw shaft areused. For example, for derotation of deformed spinal columns, screws areprovided in which a pivot movement between the tulip and the screw shaftis blocked in the medial-lateral direction (i.e., in the lateraldirection with respect to a spinal column or the associated human body),but is permitted in the cranial-caudal direction (i.e., in thelongitudinal direction or height direction with respect to the spinalcolumn) for positioning during surgery. This is achieved by making thescrew head non-circular, e.g. with lateral flattenings, and an inlay orinsert inserted into the tulip for axial application of pressure to thescrew head for fixing the relative position between the screw and thetulip is designed with corresponding lateral inner surfaces which comeinto contact with the flattenings on the screw head.

Such pedicle screws are implanted using a special instrument orinstrument set, which is usually standardized for a number of differentpedicle screws.

From the prior art, for example from US 2010/0305621 A1 and U.S. Pat.No. 9,138,261 B2, a polyaxial pedicle screw is known in each case whoseround head inserted in a receiving body is at least partially flattenedon two opposite sides, so that a bushing (inlay/insert for axialinsertion in a tulip) arranged proximally to the head has two ear-shapedaxial projections which form flat inner surfaces, are guided with theseinner surfaces on the flattened sides of the head and permit pivoting ofthe screw shaft and screw head relative to the bushing only in a planeparallel to the flat inner surfaces. Part of the head of the pediclescrew may protrude radially beyond the flattened sides to provideadditional screw diametrical support of the screw head on the receivingbody. However, this limits an achievable pivoting angle and reduces theguiding surface. Furthermore, the flanges are slightly elasticallydeformable. Accordingly, high lateral forces may not be adequatelysupported.

U.S. Pat. No. 8,048,133 B2 discloses a polyaxial pedicle screw, whoseround head is machined on two screw-diametrically opposite sides inorder to form a screw-diametrically extending cylinder, which comesproximally into contact with a corresponding surface of a pressureelement (inlay/insert) and together with this surface forms a kind ofsliding bearing, which allows rotations about a cylinder axis and blocksrotations/pivot movements about other axes. However, this blocking ofrotation or pivoting about other axes is likely to be insufficientlystable at high lateral forces and will lead to rapid wear ordeterioration of the connection.

A further document, U.S. Pat. No. 5,989,254 A, shows a polyaxial pediclescrew with a spherical head in which a cylindrical saddle surface or atransverse slot with a saddle-shaped, longitudinally curved slot base isrecessed to receive a connecting rod via which several pedicle screwsare connectable. In the case where a tulip is placed over the screw headof the pedicle screw and a connecting rod is inserted transversely intothe tulip, the saddle surface rolls off the connecting rod when thescrew head is pivoted in the tulip about an axis running diametricallyto the screw. Transverse forces are mainly transmitted via an outersurface of the connecting rod and side surfaces of the recess for thesaddle surface, i.e. via a relatively short line contact. Thisconstruction is prospectively not sufficiently stable against hightransverse forces.

In summary, known solutions in many application situations are notcapable of absorbing sufficiently high transverse loads when transversepivoting is blocked (i.e., the permitted transverse forces are too low).Furthermore, these solutions have too many individual parts of thepolyaxial pedicle screw and/or contain joints and are therefore complexto manufacture and to assemble. In particular, the internal geometry ofthe insert or insertion piece or retaining piece inserted proximally inthe screw head within the tulip is often complex. This increases themanufacturing costs, among other things. Implantation of these screwsmay also require a separate set of instruments, which further increasesthe associated manufacturing costs due to lower quantities.

SUMMARY

The present invention is based on an object to reduce or avoiddisadvantages of the prior art. In particular, a simple, stable pediclescrew with a screw head is to be provided that is pivotable within areceiving sleeve or tulip in one (single) screw-diametral direction (inparticular cranial-caudal) and is fixed in position in anotherscrew-diametrical direction (in particular medial-lateral) even withhigh transverse forces/side loads of at least 500N. Furthermore, withthe present pedicle screw, in particular the instrumentation and setscrews for fixing the screw, which should preferably be able to beloosened and tightened several times, of the same systems should beusable. In other words, the basic functions and the external geometry ofthe ‘normal’ (same system) polyaxial pedicle screw are to be retained.

The basic idea of the present invention is essentially to provideadditional support/bearing/guide areas in the pedicle screw according tothe invention between a receiving sleeve or tulip and a screw headmounted therein, which prevent transverse pivoting of the receivingsleeve or tulip relative to a screw shaft, which is integrally connecteddistally to the screw head, when a transverse load occurs. On the otherhand, these additional support areas should not impede the mobility ofthe screw head relative to the receiving sleeve or tulip of the pediclescrew about a pivot axis. According to the invention, extensionsprojecting beyond the preferably spherical screw head are provided,which form these additional support surfaces and which can be or arebrought directly into support/bearing/guide contact with the receivingsleeve. In this way, the screw head is not only secured againsttransverse rotation, as is usual with polyaxial pedicle screws, via asupport area between the screw head and an insertion sleeve/inlay/insertarranged proximal to the screw head in the receiving sleeve or tulip,with said insertion sleeve/inlay/insert serving to press against thescrew head and thereby fix its relative position in the receivingsleeve/tulip. Instead, the extensions are provided to form an additionalsupport area with the receiving sleeve/tulip to prevent such transversepivoting. Preferably, further support areas may also be provided whichare oriented in different directions, such as a cylindrical surfaceextending transversely to the longitudinal axis of the screw, in orderto always absorb the transverse load as optimally as possible.

More precisely, the object of the invention is solved by a pedicle screwof the polyaxial type, which has a screw shaft, at the proximal end ofwhich a screw head with a substantially spherical basic shape is formedin one piece, which is mounted in a receiving sleeve or tulip, in whichan insertion sleeve is inserted, which can be pressed against the screwhead to fix a relative pivot position between the receiving sleeve ortulip and the screw shaft. Two diametrically opposite pivot guide unitsor pivot restriction units are provided, each having at least oneextension projecting radially and in the longitudinal screw directionbeyond the spherical basic shape of the screw head, the extensionsforming stops on their respective, preferably partially cylindrical,radial outer surfaces. The extensions are positioned in such a way thatthey permit relative pivoting of the screw shaft and the tulip orreceiving sleeve only in the pivot plane and that, in the event ofrelative pivoting in another pivot plane (transverse pivoting), theyabut against a radially inner circumferential side of the tulip orreceiving sleeve in a supportive manner. The support areas/stops createdin this way between the screw head and the receiving sleeve/tulip meanthat very high lateral loads (at least 500N) can be transmitted andlateral pivoting can be effectively blocked. Consequently, high lateralstability is achieved through thick extensions or guide tabs.

Furthermore, no changes to the outer geometry of the overall screw arenecessary, which is why standard instruments of the same system can beused for implantation of the pedicle or bone screw. Accordingly, forsuch a pedicle screw, only internal changes to the screw constructionare made in comparison with a standard screw of the same system, withoutthe need for complex internal geometry on the insert. The insertionsleeve (insert/inlay) may be used essentially geometrically unchanged orwith only relatively minor changes for both a uniplanar screw and apolyaxial screw, which is why higher quantities and lower costs may beachieved. Furthermore, the assembly process has to be changed minimallyor even not at all, which is why no change in assembly is necessary. Thenumber of parts required (receiving sleeve/tulip/body, insertionsleeve/insert/inlay and screw head and screw shaft) is also low.Furthermore, the individual components, i.e. the screw head, theinsertion sleeve (inlay/insert) and the receiving sleeve (tulip), have asimple shape, which makes the pedicle screw easy to manufacture.

Preferably, the extensions proximally form partially cylindrical bearingsurfaces which guide the relative pivoting of the screw shaft and of thetulip or receiving sleeve in the pivot plane and which are guided in theproximal direction on frontal or distal, correspondingly partiallycylindrical recesses of the insertion sleeve in order to interact in themanner of a plain bearing. A pair of active surfaces formed in this wayserves both to guide the pivoting about the pivot axis evenly and withas little friction as possible and in order to support or block thetransverse pivoting, in particular about an axis transverse to the pivotaxis. Since the extensions are diametrically opposite each other, alever is also created between the at least two extensions or thecorresponding bearing surfaces, via which transverse forces acting onthe screw can be transmitted particularly well. Accordingly, pivoting ofthe screw head relative to the receiving sleeve/tulip about an axistransverse to the pivot axis can be effectively blocked.

It has proven expedient that the tulip or receiving sleeve has, on aninner circumferential surface, at least two diametrically oppositecavities, which are sufficiently large to accommodate the extensions inany pivot position. Without these cavities, pivoting of the screw headrelative to the receiving sleeve/tulip would not be possible or wouldonly be possible to a minor extent due to the extensions. Accordingly,the pivotability of the screw head in the receiving sleeve/tulip isensured in particular by the cavities. Preferably, the cavities and theextensions are dimensioned and positioned in such a way that theextensions can abut against the radially inner circumferential side ofthe tulip or receiving sleeve in any pivoting position. This can beachieved, for example, in that the cavities are essentially rectangular.This provides a large stop surface or contact surface in the receivingsleeve/tulip, in particular one extending in the longitudinal screwdirection, for the extensions of the screw head, whereby high transverseforces (at least 500N) can be transmitted directly and stably to thereceiving sleeve/tulip. Accordingly, an interaction between theextensions of the screw head and the receiving sleeve and, as describedabove, an interaction between the extensions of the screw head and theinsertion sleeve can effectively prevent pivoting about an axistransverse to the pivot axis.

It is advantageous if the extensions block pivoting of the screw head ina medial-lateral direction and allow pivoting in a cranial-caudaldirection. This is advantageous in particular in the field of spinalrotation. In terms of construction, this is achieved by the fact thatthe receiving sleeve or tulip and preferably the insertion sleeve has,offset in the circumferential direction by 90° with respect to theextensions arranged therein in the assembled state, preferably U-shapedslots for receiving a connecting rod via which a plurality of pediclescrews can be connected to each other. In principle, this can also beprovided in reverse for other application purposes. This means thatvarious orientations of the screw head relative to the receivingsleeve/tulip are possible. Furthermore, in the pedicle screw, it shouldpreferably be possible to pivot the screw shaft relative to thereceiving sleeve or tulip in one pivot plane by at least +/−22°,preferably at least +/−30°. This range is also based on values that areadvantageous for spinal rotation.

According to an advantageous embodiment of the invention, a diameter ofthe proximal bearing surfaces of the extensions is smaller than adiameter of the spherical basic shape of the screw head. That is, theextensions are arranged substantially crescent-shaped or ear-shaped onthe spherical basic shape of the screw head (in particular in thevicinity of its screw-diametrical outer circumference), wherein a roundpart of the crescent or ears is oriented in the proximal direction. Thediameter of the spherical basic shape of the screw head, in particularan outer contour of the entire screw head, is in this case preferablysmaller than or equal to an inner diameter of the receivingsleeve/tulip. As a result, only a few corners and edges are formed onthe screw head, which simplifies its manufacture and, if applicable,sterilization. Furthermore, the amount of machine finishing required forthe receiving sleeve/tulip is minimized.

Preferably, additional support lugs are formed on the screw head, whichare screw-diametrically offset inwards with respect to the extensionsand which project beyond the extensions in the proximal direction. Inparticular, these support lugs are intended to form proximally partiallycylindrical second bearing surfaces in order to interact with frontal,correspondingly partially cylindrical slots of the insertion sleeve inthe manner of a plain bearing. A diameter of the second bearing surfacesis larger than that of the above-described first bearing surfaces of theextensions. This means that additional surfaces may be provided forguidance and force transmission during the intended pivoting movementand transverse loads are better supported.

For example, it may be advantageous if the support lugs arescrew-radially flat on the outside and possibly also on the inside inorder to support themselves on corresponding surfaces of the slots toblock pivoting in the other pivot plane or the transverse pivoting. Thatis, the support lugs form a screw-diametrically flat surface on thescrew head to provide an additional radial support area or guide areafor transmitting lateral loads.

According to a further advantageous embodiment of the invention, theextensions may project screw-diametrically outwards and preferablyproximally beyond an outer diameter of the spherical base body of thescrew head. In other words, this screw-diametric widening of the screwhead provides a lever between the two extensions or the bearing surfacesformed by them. Accordingly, even greater transverse loads can besupported via the bearing surfaces of the extensions of the screw headthan according to the afore-described first advantageous embodiment.

Furthermore, an outer contour of the entire screw head (i.e., thespherical base body and the extensions projecting screw-diametricallybeyond it) may be larger than the inner diameter of the tulip orreceiving sleeve. Accordingly, it is useful if the cavities are formedby an undercut recessed in an inner circumferential wall of thereceiving sleeve/tulip, which is preferably rectangular, furtherpreferably square, in order to be able to accommodate the extensions. Inprinciple, other configurations are also possible. In the case of asquare undercut, however, it may be advantageous for the sleeve to bemounted rotated by 90° to allow the screw head to be moved in the samereceiving sleeve/tulip in different (pivot) directions.

In particular, the extensions are supported in the undercut formed bythe cavities in the receiving sleeve or tulip both in thescrew-diametrical direction outwards and in the longitudinal screwdirection. In other words, the cavities are embedded in an innercircumferential surface of the receiving sleeve/tulip in such a way thatthey form stops or support surfaces both in the screw-diametricaldirection and in the longitudinal screw direction. Accordingly, in thisembodiment, the cavities not only guide or support the extensions of thescrew head in the screw-diametrical direction, but also guide or supportthem in the longitudinal screw direction. This means that all relativemovements of the screw head are guided or supported both via theinsertion sleeve (inlay/insert) and via the receiving sleeve/tulip, andthe loads to be transmitted are distributed. Accordingly, the stiffnessof the support of the screw head is further increased in the case oftransverse loads.

Preferably, the extensions are each screw-radially flat on the insideand are guided on opposite surfaces of the recesses. This means that inthis configuration the extensions form both the stops directed radiallyor screw-diametrically outwards and the guide surfaces directed radiallyor screw-diametrically inwards. If required, these may also assumeadditional support functions. The extensions are therefore designed toprovide both the advantages of the crescent-shaped extensions and theadvantages of the support lugs described above. As a result, therigidity of the support of the screw head under transverse loads can befurther increased.

Alternatively or additionally, in any of the aforementioned embodimentsof the invention, a part of the screw head may be cylindrically milledoff to provide a further bearing surface on the screw head, whichinteracts as a sliding bearing with corresponding bearing surfacesformed on the front side of the insertion sleeve or on the insert/inlay.

In summary, the object underlying the invention can be achieved byattaching lateral lugs to the head end of a bone screw/pedicle screw andcorresponding interior spaces to a receiving sleeve or body or tulip,and by corresponding cavities to the insert as well. The lugs aresupported both on the insert and on the body in the lateral directionand block movement in the lateral direction while at the same timeallowing free movement in the cranial-caudal direction. The screw headand the lugs form a unit and have a cylindrical contour on the outside.Due to the design according to the invention, no additional connectingelements such as pins, axes and the like are necessary. For assembly ofthe entire pedicle screw (according to one embodiment, see the thirdembodiment described below), the bone screw may be assembled withouttools via a rotational movement of the screw relative to the body,despite an outer diameter of the lugs that is larger than the innerdiameter of the body. Due to the larger lug diameter, the screw is notonly supported radially on the body during use, but also axially via theend faces of the lugs.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

The present invention is described below based on preferred embodiments,which are described below by way of illustration. It should be notedthat features of different embodiments may be combined and variousmodifications may be made without departing from the scope of protectionof the invention.

FIG. 1 shows a perspective, partial sectional view of the pedicle screwaccording to a first embodiment of the invention;

FIG. 2 shows a receiving sleeve or tulip in the longitudinal sectionaccording to the first embodiment;

FIG. 3 shows a screw head according to a second embodiment of theinvention;

FIG. 4 shows an insertion sleeve according to the second embodiment ofthe invention;

FIG. 5 is a longitudinal sectional view of the assembled pedicle screwaccording to the second embodiment of the invention;

FIG. 6 is a cross-sectional view of the receiving sleeve according tothe second embodiment in the region of the cavities;

FIG. 7 is a side view of a screw head according to a third embodiment ofthe invention;

FIG. 8 shows an insertion sleeve according to the third embodiment ofthe invention;

FIG. 9 shows a partial perspective sectional view of the pedicle screwaccording to the third embodiment; and

FIG. 10 shows a longitudinal sectional view of the pedicle screwaccording to the third embodiment of the invention.

DETAILED DESCRIPTION

FIG. 1 shows a perspective, partial sectional view of a pedicle screw 1according to the invention in an angled state. The pedicle screw 1 has atulip or receiving sleeve 2, which is longitudinally cut in thisrepresentation and whose inner circumference is chronically narrowed ata distal end to form a contact surface 3 for a spherical screw head 4 ofthe pedicle screw 1. The pedicle screw 1 was inserted into the receivingsleeve 2 from the proximal direction in such a way that the sphericalscrew head 4 rests distally against the contact surface 3 and a screwshaft 5 integrally formed therewith projects distally from the receivingsleeve 2, so that the screw head 4 and the screw shaft 5 can be pivotedrelative to the receiving sleeve 2.

The screw head 4 is also designed to be at least partially sphericalproximally and also has two extensions 6 on screw-diametrically oppositesides, which extend from an outer circumference of the screw head 4 in acrescent shape in the proximal direction. I.e., distally orientedsurfaces of the extensions 6 form bearing surfaces 6 a, which serve tosupport the screw head 4 on an insertion sleeve 8 (inlay/insert). In thepedicle screw 1, these bearing surfaces 6 a or their diameter define apivot axis S about which the screw head 4 can be pivoted in relation tothe receiving sleeve 2. In order to illustrate this, the pedicle screw 1in FIG. 1 is shown in a pivoted state, which is made clear by thelongitudinal axes of the screw shaft A and the receiving sleeve Bintersecting at a pivot axis, i.e. the center axis of the bearingsurfaces 6 a. In addition, the extensions 6 form a preferablysemi-cylindrical radial outer surface or a preferably semi-cylindricalradial stop 6 b, which serve as a support surface for supporting thescrew head 4 on the receiving sleeve 2.

The insertion sleeve 8 partially forms a hollow-spherical receivingsurface 9 on its distal side for receiving the round screw head 4, whichis not shown here but essentially corresponds to the correspondingreceiving surface 9 of FIG. 4 and FIG. 8. Furthermore, a frontal, distaledge of the insertion sleeve 8 is not continuous, but has twodiametrically opposite recesses 10, which correspond to thecrescent-shaped or ear-shaped extensions 6, or more precisely, theirbearing surfaces 6 a. The recesses 10 interact with the bearing surfaces6 a in the manner of a plain bearing to ensure pivotability of thereceiving sleeve 2 and the screw head 4.

At a proximal region of the insertion sleeve 8, it has two lateralbulges 11 offset by 90° to the recesses 10, which are received incorresponding saddle surfaces 12 within the receiving sleeve 2 in apredetermined position. The insertion sleeve 8 furthermore has asemi-cylindrical opening 13, which extends through the bulges 11 forreceiving a connection rod, via which two pedicle screws 1 can beconnected to each other during implantation or surgery. The receivingsleeve 2 furthermore has, at the same angular position as the saddlesurfaces 12, two U-shaped slots 14 introduced from the proximal side,into which the connection rod is insertable. A thread 15 is provided ata proximal area within the receiving sleeve 2, into which a set screwcan be screwed for tensioning the pedicle screw 1 and the connectionrod.

FIG. 2 shows a longitudinal section of the receiving sleeve 2 accordingto the first embodiment of the invention. In addition to the featuresalready described above, it is apparent from this view that in an innercircumferential region of the receiving sleeve 2, in which the screwhead 4 is received, two cavities 16 (only one shown here) are provideddiametrically opposite each other. These cavities 16 serve to receivethe extensions 6 in order to ensure the mobility or pivotability of thescrew head about the pivot axis S. At the same time, the cavities 16form a support surface or stop surface 16 a in order to support thescrew head 4 directly on the receiving sleeve 2 via the extensions 6, ormore precisely, via the radial stops 6 b.

The second embodiment of the invention shown in FIG. 3 to FIG. 6corresponds to a large extent to that of the first embodiment, which iswhy only their differences are discussed below and the same referencesigns are used for similar features.

FIG. 3 shows a perspective view of the screw head 4 according to thesecond embodiment of the invention. Here it can be seen that the screwhead 4 has a proximal front surface 17, in which, for example, a toolreceptacle or a channel for pressing in bone cement may be embedded.Such a front surface 17 may also be provided in the first embodiment.Furthermore, in addition to the extensions 6, the screw head 4 hassupport lugs 18 which are offset radially inwards. These support lugs 18extend proximally beyond the proximal front surface 17 of the screw head4 and also form at their proximal end lateral surfaces of the cylinderor second bearing surfaces 19 in order to form a further guide surfaceor support surface or bearing surface for guiding or supporting orbearing on the insertion sleeve 8. In the screw-diametrical direction,the support lugs 18 are formed flat on both sides.

FIG. 4 shows an insertion sleeve 8 according to the second embodiment ofthe invention. In addition to the receiving surface 9 and the recesses10, this insertion sleeve distally has embedded slots 20 which areoffset radially inwards relative to the recesses 10. This means that aposition of the slots 20 corresponds to a position of the support lugs18. Accordingly, in the distal direction, the slots 20 form an innercylindrical surface, which interacts with the support lugs 18, moreprecisely, with the lateral cylinder surface or the second bearingsurface 19 of the support lugs 18, in the manner of a plain bearing.

FIG. 5 shows an assembled pedicle screw 1 according to the secondembodiment in a longitudinal section. It can be seen how the extensions6 of the screw head 4 rest screw-diametrically on the receiving sleeve2, more precisely, on the support surfaces or stop surfaces 16 a of thecavities 16, in order to be supported in the screw-diametricaldirection. Furthermore, it can be seen that the bearing surfaces 6 a ofthe extensions 6 of the screw head 4 and the recesses 10 of theinsertion sleeve 8, as well as the second bearing surfaces 19 of thesupport lugs 18 and the slots 20 of the insertion sleeve 8, abut againsteach other in order to support and guide the screw head 4 inlongitudinal screw direction against the insertion sleeve 8.Furthermore, it can be seen that the support lugs 18 are supported atleast with the radially outer flat side 21 against corresponding sidewalls of the slots 20 in order to increase the stability of the pediclescrew 1 against pivoting in a direction transverse to the intended pivotaxis S (transverse pivoting). Furthermore, according to this embodiment,two diametrically opposite stop projections 22 are provided in thereceiving sleeve 2, offset by 90° to the slots 14, which serve to holdthe insertion sleeve 8 provisionally (i.e., as long as the connectionrod has not yet been inserted and the set screw has not yet been screwedin) in the receiving sleeve 2.

FIG. 6 shows a cross-section of the receiving sleeve 2 in the area inwhich the screw head 4 is received. In particular, this illustrationshows the shape of the cavities 16 in which the extensions 6 of thescrew head 4 are pivotably received. The cavities 16 are substantiallyrectangular in cross-section, whereby the radial stops 6 b of theextensions 6 of the screw head abut the support surfaces or stopsurfaces 16 a formed by the cavities 16 in any pivoted position. Thecavities 16 in the first embodiment also have such a shape, but may beformed smaller.

FIG. 7 shows a side view of a screw head 4 according to a thirdembodiment of the invention. Compared with the pedicle screw accordingto the first and second embodiment, the pedicle screw 1 according tothis embodiment is designed to transmit particularly high forces and istherefore designed to be particularly stable. In the following, thespecial features and differences of this embodiment compared to theother two embodiments, in particular the first embodiment, arediscussed, wherein the same reference signs are used for similarfeatures.

This screw head 4 is also essentially ball-shaped/spherical and forms aproximal front surface 17, in which a tool receptacle or a channel maybe embedded. Furthermore, a proximal half of the spherical surface ofthe screw head 4 is provided with cannelures or ribs 23. Due to theseforms, the screw head 4 is not freely pivotable about the pivot axis Srelative to the insertion sleeve 8, but can be locked in a specificposition or pivot position via the ribs 23. Furthermore, the extensions6 are very large. That is, they project in the screw-diametricaldirection beyond the outer circumference of the spherical base body ofthe screw head 4, so that a screw-diametrical extension of theextensions 6 is greater than the diameter of the spherical base body ofthe screw head 4. In addition, the extensions 6 project in thelongitudinal screw direction beyond the proximal front surface 17 of thescrew head 4. In this way, the extensions 6 may form additional supportsurfaces on their radial inner side, which, similar to the support lugs18 of the second embodiment, are supported on corresponding surfaces ofthe insertion sleeve 8 in order to achieve additional stability of thepedicle screw against transverse pivoting. If required, the extensions 6may also be used to clamp/span the insertion sleeve 8 between them.

Radially within the extensions 6, the spherical screw head 4 iscylindrically recessed, e.g. by milling, so as not to impede a pivotingmovement of the receiving sleeve 2 between the extensions 6 of the screwhead 4. If required, this may provide a further bearing surface 24 inaddition to the bearing surfaces 6 a formed by the extensions 6. Insummary, the third embodiment is essentially a combination of the firstand second embodiments, which provides a particularly high degree ofstability against transverse pivoting.

FIG. 8 shows a perspective view of the insertion sleeve 8 according tothe third embodiment of the invention. It is clearly visible that therecesses 10 of the insertion sleeve 8 are significantly deeper thanthose of the first and second embodiment, so that the large extensions 6have room therein. Furthermore, the recesses 10 are offset radiallyoutwards, creating a step in each case. The radially inner, more distalpart of each step is also cylindrically recessed so as not to impede apivoting movement of the receiving sleeve 2 between the extensions 6 ofthe screw head 4. In addition, if required, additional bearing surfaces25 may be formed by these more distal parts of the steps, which areadapted to interact with the bearing surfaces 24 of the screw head 4 inthe manner of a plain bearing.

FIG. 9 shows a perspective, partial sectional view of the assembledpedicle screw 1 according to the third embodiment of the invention. Itis clearly visible how the receiving surface 9 of the insertion sleeve 8sits on the ribs 23 of the screw head 4 and how the extensions 6 engagein the recesses 10 of the insertion sleeve 8 to form pairs of activesurfaces both via the bearing surfaces 6 a of the extensions 6 with therecesses 10 of the insertion sleeve 8 in the longitudinal screwdirection and via the radially inner support surfaces of the extensions6 with the steps formed by the recesses 10 in the screw-diametricaldirection.

FIG. 10 shows a longitudinal section of an assembled pedicle screwaccording to the third embodiment, which reveals a further specialty ofthe present embodiment. It can be seen that the cavities 16 form anundercut in the receiving sleeve 2. That is, the cavities 16 provide notonly the screw-diametrically oriented support surface or stop surface 16a, but also a proximal, screw-longitudinally oriented additional supportsurface 16 b, against which the bearing surfaces 6 a of the extensions 6of the screw head 4 come into contact. This ensures that the screw head4 is also supported in the longitudinal screw direction not only on theinsertion sleeve 8 but also directly on the receiving sleeve 2. Thismeans that according to this embodiment, the screw head 4 is directlysupported by the receiving sleeve 2 and the insertion sleeve 8 both inthe proximal and in the screw-diametrical direction and maximumstability against transverse pivoting is achieved. The cavity 16 may besquare in cross-section so that the screw head can also be usedtransversely and the same receiving sleeve can be used for differentlyoriented polyaxial pedicle screws.

1. A polyaxial pedicle screw comprising: a screw shaft; a screw headwith a substantially spherical shape formed in one piece at a proximalend of the screw shaft; a receiving sleeve or tulip in which the screwhead is mounted; an insertion sleeve inserted into the receiving sleeveor tulip, the insertion sleeve configured to be pressed against thescrew head to fix a relative pivot position between the receiving sleeveor tulip and the screw shaft; and two diametrically opposite pivot guideunits or pivot restriction units, which each have at least one extensionprojecting radially and in a longitudinal screw direction beyond thesubstantially spherical shape of the screw head, wherein the extensionsform stops on their respective radial outer surfaces, and are positionedin such a way that they permit relative pivoting of the screw shaft andthe tulip or receiving sleeve only in a first pivot plane and that, inthe event of relative pivoting in a second pivot plane, they aresupported to be stopped at a radially inner circumferential side of thetulip or receiving sleeve.
 2. The polyaxial pedicle screw according toclaim 1, wherein the extensions proximally form partially cylindricalbearing surfaces which guide the relative pivoting of the screw shaftand the tulip or receiving sleeve in the first pivot plane and which areguided in a proximal direction on frontal or distal, correspondinglypartially cylindrical recesses of the insertion sleeve.
 3. The polyaxialpedicle screw according to claim 2, wherein the tulip or receivingsleeve has, on an inner circumferential surface, at least two cavitiesthat are diametrically opposed and sufficiently large to accommodate theextensions in any pivoted position.
 4. The polyaxial pedicle screwaccording to claim 1, wherein pivoting of the screw shaft relative tothe receiving sleeve or tulip in the first pivot plane by at least+/−22°.
 5. The polyaxial pedicle screw according to claim 1, wherein thereceiving sleeve or tulip has, offset in the circumferential directionby 90° with respect to the extensions, slots for receiving a connectingrod, via which the polyaxial pedicle screw is connectable to one or moreother pedicle screws.
 6. The polyaxial pedicle screw according to claim1, wherein a diameter of the proximal bearing surfaces of the extensionsis smaller than a diameter of the substantially spherical shape of thescrew head.
 7. The polyaxial pedicle screw according to claim 1, whereinthe screw head comprises support lugs which are screw-diametricallyoffset inwards with respect to the extensions and which project beyondthe extensions in the proximal direction.
 8. The polyaxial pedicle screwaccording to claim 7, wherein the support lugs form proximally partiallycylindrical second bearing surfaces supported at frontal,correspondingly partially cylindrical slots of the insertion sleeve. 9.The polyaxial pedicle screw according to claim 1, wherein the supportlugs are screw-radially flat on the outside and supported oncorresponding side surfaces of the slots to block pivoting in the secondpivot plane.
 10. The polyaxial pedicle screw according to claim 3,wherein the extensions project screw-radially outwards beyond an outerdiameter of the substantially spherical shape of the screw head.
 11. Thepolyaxial pedicle screw according to claim 10, wherein the cavities areformed by an undercut recessed in an inner circumferential wall of thereceiving sleeve or tulip.
 12. The polyaxial pedicle screw according toclaim 11, wherein the extensions are supported in the undercut formed bythe cavities in the receiving sleeve or tulip both screw-diametricallyoutwards and in the longitudinal screw direction.
 13. The polyaxialpedicle screw according to claim 10, wherein the extensions are eachflat screw-radially on an inside and are guided on correspondingsurfaces of the recesses.
 14. The polyaxial pedicle screw according toclaim 1, wherein the radial outer surfaces of the extensions extend inthe longitudinal screw direction.
 15. The polyaxial pedicle screwaccording to claim 1, wherein the radial outer surfaces form part of alateral surface of a cylinder extending in the longitudinal screwdirection.
 16. The polyaxial pedicle screw according to claim 1, whereinthe radial outer surfaces are respectively supported at flat lateralwall surfaces formed on the radially inner circumferential side of thetulip or receiving sleeve.
 17. The polyaxial pedicle screw according toclaim 1, wherein the radial outer surfaces extend essentially parallelto the first pivot plane.
 18. The polyaxial pedicle screw according toclaim 9, wherein the support lugs are screw-radially flat on the insideand are supported on the corresponding side surfaces of the slots toblock pivoting in the second pivot plane.
 19. The polyaxial pediclescrew according to claim 10, wherein the extensions project proximallybeyond the outer diameter of the substantially spherical shape of thescrew head.
 20. The polyaxial pedicle screw according to claim 11,wherein the undercut has a rectangular cross-section.